//对象转查询字符串
function objToSearchString(obj) {
  var str = ``
  for (var key in obj) {
    str += `${key}=${obj[key]}&`
  }
  return str.slice(0, str.length - 1)
}
function ajax(method, url, params) {
  //new一个promise对象并且返回
  return new Promise((resolve, reject) => {
    var xhr = new XMLHttpRequest()
    xhr.open(method, method == 'get' ? url + '?' + objToSearchString(params) : url)
    xhr.onreadystatechange = function () {
      if (xhr.readyState == 4) {
        resolve(JSON.parse(xhr.responseText))
      }
    }
    xhr.setRequestHeader('content-type', 'application/x-www-form-urlencoded');
    xhr.send(method == 'get' ? null : objToSearchString(params))
  })
}